A technique has conventionally been known for a video display apparatus including a liquid crystal display, etc., to control light emission brightness of a backlight light source depending on the ambient luminance. Such a video display apparatus includes a luminance sensor to detect the ambient luminance, and controls the light emission brightness of the backlight light source depending on the luminance around the video display apparatus detected by the luminance sensor. For example, when the luminance around the liquid crystal display is increased, the visibility not overwhelmed by the ambient light can be acquired by increasing the light emission brightness of the backlight light source following the increase of the luminance (see, e.g., Patent Document 1).
A type of the above video display apparatus is prevailing that uses an LED backlight to illuminate the display panel. The LED backlight has an advantage that local dimming is available. The “local dimming” refers to control of LED light emission for each of areas acquired by dividing the backlight into plural areas, depending on the video signal for each area. For example, the control is enabled such that the light emission of the LED is suppressed for a dark portion in the screen and the light emission of the LED is enhanced for a light portion therein. Thereby, the power consumption of the backlight can be reduced and the contrast of the display screen can be improved.
For example, exemplary control based on the conventional local dimming is depicted in FIG. 9. In this case, the backlight is divided into eight areas and the brightness of the LEDs is controlled depending on the maximal gray scale value of the video signal that corresponds to each of the areas. It is assumed that the maximal gray scale value of the video signal for each area is in the state depicted in (A) of FIG. 9. Each of “A” to “H” denotes an area No., and the number therebeneath is the maximal gray scale value in the area. For example, the brightness of the LED of each area based on the local dimming is depicted in (B) of FIG. 9. The brightness of the LED is controlled for each area depending on the video signal for the area. In this example, the video image is relatively dark in an area where a maximal gray scale value of the video signal is low and, therefore, the brightness of the LEDs is lowered to reduce the black float, to improve the contrast and to save the power consumption of the LEDs. In this case, the maximal brightness in each area is limited to the brightness acquired when all the LEDs of the backlight are lit up at the duty of 100% (for example, 450 cd/m2).